The present invention relates to the field of blades such as those used in fans of turboprops or designed to form propeller vanes for aircraft.
These blades are generally made of metal material. If blades made of metal material have good mechanical resistance, they still have the disadvantage of having a relatively large mass.
To produce lighter blades, it is known to make blades of composite material, that is, by making at least one part of the pieces of the blade from a fibrous reinforcement densified by a resin matrix.
The technique generally used consists of forming a skin about a core or longeron made of metal material also integrating the stilt and the blade foot, by stacking unidirectional pre-impregnated folds or layers (draping) on the longeron, the whole then being placed in a mould by variously orienting the successive folds, prior to compacting and polymerisation by autoclave. An example of an embodiment of a propeller blade according to this technique is especially described in document U.S. Pat. No. 6,666,651.
This type of blade structure breaks down into two sub-structures, specifically the skin which corresponds to the first millimeters of material under the intrados and extrados surfaces and the rest of the structure which is composed essentially of the longeron, the stilt and the blade foot. Most often, the skin ensures only the aerodynamic and aeroelastic behaviour of the blade whereas the rest of the structure ensures the structural behaviour of the blade (resistance to impact, overspeeds, etc.).
However this type of structure is not optimal in that concentrations of stresses and irregularities in deformations appear between the skin made of composite material and the rest of the structure, preventing even transmission of forces exerted on the skin towards the rest of the structure intended to ensure the structural behaviour of the blade. It is therefore very difficult in this case to avoid creating weakness zones in the blade and consequently obtaining reliable mechanical behaviour.
To especially improve resistance to impact and reduce the risks of delaminating in composite blades, blades have been manufactured by three-dimensional weaving of a fibrous preform and densification of the preform by an organic matrix as described in document EP 1 526 285. This process produces blades having very high mechanical resistance. But, according to this technique the whole of the structure of the blade, specifically the skin and the rest of the structure (longeron, stilt and foot) is made from one and the same continuously woven fibrous reinforcement, but as a variant to armour as a function of the parts of the blade to be manufactured.
This type of production does not structurally dissociate the skin from the rest of the structure of the blade, which reduces its capacity to be arranged inside the blade, such as for example integration of cooling ducts, deicing elements and/or introduction of low-density material to lighten the overall mass of the blade.